coverage: Never decrease the start column to 0

Line/column numbers are 1-based, so if the start column is already 1 at this
point, we would decrement it to 0 and confuse `llvm-cov`.

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -277,7 +277,7 @@ fn make_code_region(
     if span.hi() == span.lo() {
         // Extend an empty span by one character so the region will be counted.
         if span.hi() == body_span.hi() {
-            start_col = start_col.saturating_sub(1);
+            start_col = start_col.saturating_sub(1).max(1);
         } else {
             end_col = start_col + 1;
         }